This invention generally relates to correction of non-uniformities in a printhead that directs exposure energy onto a medium and more particularly relates to a printhead having multiple exposure elements corrected for non-uniformities, a method for correcting non-uniformity of such exposure elements, and an apparatus for non-uniformity correction of a print head.
Recent advances in printer technologies and related technologies, such as digital photography, have led to increased use of high quality printers. The problem of non-uniform density output from a printer is a well-known problem in the printer art. Non-uniformity is particularly problematic in high-quality color printers, such as in the photofinishing arts, where it is important to be able to faithfully reproduce subtle changes in shading and gradation or flat fields having the same density. Non-uniform response of a printhead can cause unacceptable anomalies such as streaking and banding, which can easily render a print unacceptable, or at least disappointing, for its intended audience.
Factors that contribute to printer non-uniformity vary, depending on the specific print technology. With a thermal printhead, for example, where resistive print elements are aligned along a writing surface, slight mechanical irregularities or tolerance build-up can cause some elements to be more effective in transferring heat, than others. With a printhead that scans optically, such as a CRT printhead, optical aberrations or fringe effects can mean that light power is less effectively distributed at the extreme edges of the scan pattern than it is in the center of a scan line. In a photofinishing system that uses an array of light-emitting exposure elements, such as a Micro Light Valve Array (MLVA) using lead lanthanum zirconate titanate (PLZT) light valves (sold for example as the model QSS-2711 Digital Lab System manufactured by Noritsu Koki Co., located in Wakayama, Japan), individual elements in the array may emit varying intensities of light. The continued development of digital solutions for image scanning and exposure make the problem of achieving print uniformity particularly important.
Conventional solutions for correcting non-uniformity of printheads having multiple print elements are directed to obtaining an output power measurement for each print element of the array and adjusting a variable drive energy (typically voltage, current, or pulse width) for each array print element in order to compensate for differences in the output power measurement.
U.S. Pat. No. 5,661,544 (Ishikawa et al.) discloses measuring print density of a target medium using a scanning device, such as a flatbed scanner. Exposure correction data are then converted to electronic control signals for the individual print exposure elements. Notably, the method disclosed in this patent does not directly measure exposure energy of print elements but measures the effects of exposure energy when applied to a photosensitive medium. Accordingly, this method requires compensation for variability of print media sensitivity, print development chemistry, and scanner response characteristics.
U.S. Pat. No. 5,684,568 (also to Ishikawa et al.) discloses direct measurement of output power of exposure elements of a PLZT printer. A sensor is scanned across the PLZT array, to measure the light output of each individual array component. Compensation factors can then be computed for individual components in the PLZT array.
U.S. Pat. No. 5,016,027 (Uebbing) discloses a light detector consisting of an array of photodiodes that allows continual measurement of exposure power from LED sources of an LED printhead. The light detector is disposed along an edge of the beam path, but before any focusing element used for exposure of the photosensitive medium. The light detector measures the relative output power of each LED exposure element and actual exposure power is derived by applying compensation for measured losses in the focusing optics.
U.S. Pat. No. 5,640,190 (Bollansee et al.) discloses measuring output power for a group of LED exposure elements and for individual exposure elements. In addition, measurements of output density are obtained. A compensation factor is computed based on the measurements to correct for non-uniformity of the exposure elements.
It is also known to store correction information in a memory that is coupled to a printhead itself. As an example, U.S. Pat. No. 4,827,279 (Lubinsky et al.) discloses storing a correction table for adjusting pulse count in a memory coupled to a thermal printhead. Similarly, U.S. Pat. No. 5,684,568 (Ishikawa et al.) discloses storing a uniformity correction table in a read/write memory coupled to a PLZT printhead.
It is an object of the present invention to provide an improvement to a printer for non-uniformity correction and a method and apparatus for non-uniformity correction in a printhead having a plurality of exposure elements.
A first aspect of the invention is a printhead assembly for imaging onto a medium, the printhead assembly comprising a printhead having a plurality of exposure elements capable of emitting exposure energy into an area, a memory coupled to the printhead and containing a compensation value for each of at least some of the exposure elements. The compensation values are determined by obtaining a plurality of positional output power measurements corresponding to one of a set of predefined spaced-apart positions within the area to determine a spatial characteristic of exposure energy emitted by the corresponding exposure element.
A second aspect of the invention is an apparatus for obtaining a spatial profile of exposure output power from an exposure element in a printhead comprising a plurality of exposure elements. The apparatus comprises a sensor configured to obtain positional output power measurement for the exposure elements corresponding to a portion of the exposure output power of the corresponding exposure element emitted at each one of a plurality of predefined positions, a translation apparatus for configuring the sensor for measurement at each one of the plurality of predefined positions for the exposure element to thereby permit the sensor to obtain a sequence of the positional output power measurements for a corresponding one of the exposure elements, control logic programmed to accept the positional output power measurements from the sensor and to generate spatial profile information using the sequence of the positional output power measurements, and a memory operatively coupled to the control logic and storing the spatial profile information for the corresponding one of the exposure elements.
A third aspect of the invention is a method for compensating for non-uniformity of an exposure element in a printhead comprising a plurality of exposure elements which emit exposure energy into an area. The method comprises the steps of obtaining a sequence of output power measurements for an exposure element, each the output power measurements in the sequence corresponding to a predetermined position within the area, deriving a correction value based on the sequence of output power measurements, storing the correction value for the exposure element, and applying the correction value for controlling the exposure element.
A fourth aspect of the invention is a method for printing using a printhead that employs a plurality of exposure elements that emit exposure energy within an area. The method comprises the steps of obtaining a spatial profile compensation value for an exposure element from a sequence of measurements made at successive positions within the area, and deriving exposure element compensation values based the spatial profile.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.